The present invention relates generally to accelerated weathering test devices of the type used to expose test specimens of exterior coatings such as paints and finishes, as well as fabrics and other materials to solar radiation and other weathering effects on an accelerated basis, and more particularly, to such an accelerated weathering test device adapted to dynamically control a test specimen temperature.
Manufacturers of exterior coatings, such as paints and finishes, as well as plastics and other components which tend to degrade under exposure to solar radiation and other weathering effects, often want to know how such products will perform following years of exposure. However, such manufacturers typically require such information in a much shorter time than it would take to expose such materials to weathering effects under normal conditions. Accordingly, accelerated weathering test devices have been developed which accelerate the effects of weathering due to outdoor exposure in a much shorter time so that manufacturers need not actually wait five or ten years in order to determine how their products will hold up after five or ten years of actual outdoor exposure.
One known accelerated weathering test device is disclosed in U.S. Pat. No. 2,945,417, issued to Caryl et al. The aforementioned test device includes a Fresnel-reflecting solar concentrator having a series of ten flat mirrors which focus natural sunlight onto a series of test specimens secured to a target board measuring approximately five (5) inches wide by fifty-five (55) inches long. The Fresnel-reflecting solar concentrator directs solar radiation onto the target board area with an intensity of approximately eight suns. Both the bed which supports the mirrors of the solar concentrator, and the target board, are supported by a frame which can be rotated to follow daily movements of the sun. A solar tracking mechanism responsive to the position of the sun, controls the operation of an electric motor that is used to rotate the test apparatus to follow movements of the sun. The axis of rotation of the test machine is oriented in a north-south direction, with the north elevation having altitude adjustment capability to account for variation in the sun""s altitude at various times during the year. Such known testing devices are also provided with an air tunnel mounted above the target board. An air deflector causes air escaping from the air tunnel to be circulated across the test specimens mounted to the target board to prevent the test specimens from overheating due to the concentrated solar radiation to which they are exposed. The amount of air is controlled by the dimension of the gap between the deflector and the specimen. A squirrel cage blower communicates with the air tunnel for blowing cooling ambient air there through. In addition, water spray nozzles are provided proximate to target board for wetting the test samples at periodic intervals to simulate the weathering effects of humidity, dew, rain, etc.
Another known accelerated weathering test device is disclosed in U.S. Pat. No. 4,807,247 issued to Robins, 111. The aforementioned test device includes all the structure previously described above with respect to the ""417 patent and further includes a system for maintaining a uniform, constant test specimen temperature during daylight hours despite variations in ambient air temperature and variations in solar radiation intensity.
The system includes a temperature sensor mounted to the target board for exposure to the concentrated solar radiation and for generating an electrical signal indicative of the temperature of the test specimen mounted to the target board. The system further includes a control mechanism electrically coupled to the temperature sensor and responsive to the electrical signal generated thereby for selectively controlling the application of electrical power to the electrical motor included within the air circulation system. In this manner, the control mechanism serves to vary the speed of the electric motor and thereby control the flow rate of cooling ambient air circulating across the target board so that the temperature of the test specimen remains constant at the desired set point.
When the sensed temperature of the test specimen increases, the control mechanism increases the speed of the blower motor to circulate more cooling ambient air across the target board in order to lower the temperature of the test samples back to the desired set point. Similarly, if the sensed temperature of the target samples drops below the desired nominal temperature, the control mechanism decreases the speed of the blower to permit the test samples to warm up back to the desired set point.
The temperature control mechanism also includes a user operable adjustment mechanism, in the form of the control knob, for allowing a user to set a static, desired target specimen temperature. A bypass switch is also provided for allowing the user to operate the test device in the controlled temperature-mode as described above, or in an uncontrolled mode wherein the blower motor operates at a constant speed.
Standardized testing methods have been developed for operating accelerated weathering test devices of the type described above. The American Society for Testing and Materials (ASTM) has issued standards G90, E838, D4141, D3105, D3841, D5105, E1596 and D4364 covering the testing procedures and the operating parameters for conducting such outdoor accelerated weathering tests. Other standards and appraisals have also been developed and specified by the Society of Automotive Engineers (SAE), Ford, International Standards Organization (ISO), American National Standards Institute (ANSI), Japan Industrial Standard (JIS), namely, SAE J576, SAE J1961, Ford EJB-M1J14-A, Ford EST-M5P11-A, ISO 877, ANSI/NSF 54, JIS Z 2381 and MIL-T-22085D.
Apart from outdoor accelerated weathering test devices of the type described above, other test devices are also known which utilize an artificial source of radiation to expose the test specimens. An example of such a test device is disclosed in U.S. Pat. No. 3,664,188 issued to Kockott. While such test devices have the advantage of permitting precise control over radiation intensity, temperature and humidity, such test devices have often failed to duplicate the actual light spectrum of natural sunlight to which the specimens under test will actually be exposed in everyday use. It has been acknowledged and recognized by those of skill in the art that natural sunlight and artificial sunlight test apparatus are distinct from one another and provide different sets of empirical data.
Outdoor accelerated weathering test devices of the type described above in regard to U.S. Pat. Nos. 2,945,417 and 4,807,247, have the advantage of using natural sunlight, and hence the specimens under test are exposed to the actual spectrum of sunlight. However, disadvantages of outdoor accelerated weathering test devices have been discovered. One such disadvantage is that test results obtained from an outdoor accelerated weathering test apparatus without temperature control are not repeatable or reproducible. The blower motor used to circulate cooling air across the test specimens operates at a constant speed and generates a constant flow rate of cooling across the test specimens. Accordingly, the temperature of the test specimens cannot be controlled and is subject to random constant changes in the local ambient air temperature and solar radiation intensity. Further, it has also been discovered that the changes in the temperature of the test specimens can alter the rate of weathering which occurs. For example, test specimens tend to degrade faster in the summer than in the winter due to nominally higher test specimen temperatures in the summer as a result of both higher average ambient temperature and greater solar radiation intensity. Therefore, in order to obtain repeatable and reproducible test results, the temperature of the test specimens must be controlled.
Another disadvantage is that test results obtained from an outdoor accelerated weathering test apparatus having a static temperature control are not repeatable or reproducible. Further the test specimens are not controlled to reproduce a natural weathering function when the set point is constant. This is a critical disadvantage because the manner in which materials degrade is defined by their end-use application and environment. The test results are better than an uncontrolled apparatus. However, materials in end-use applications encounter temperature fluctuations that are best represented by complex functions. A simple diurnal material temperature cycle is a complex function of ambient temperatures, solar irradiance, material heat capacity, material re-radiance losses, conductive heat losses, convective heat losses, etc. Diurnal cycles are also super imposed upon longer time-scale cycles (season, annual, etc.) as well as intermittent random variables such as intermittent clouds, rain, dew, etc. Prior devices which are uncontrolled or maintain the test specimens at a manually set static temperature, do not account for the above variables. Rather, the temperature control for prior art accelerated weathering test apparatus simply operates at a desired, but static set temperature.
Therefore, there exists a need for a dynamically controlled accelerated weathering test apparatus which overcomes the disadvantages of prior art devices and simulates the complex temperature cycles materials encounter in end-use environments described above.
One aspect of the present invention is directed to an accelerated weathering test apparatus of the type used to concentrate solar radiation upon target specimens where the apparatus is adapted to dynamically control a target specimen temperature to simulate a complex temperature cycle of a material end-use application. The apparatus includes a target board for supporting at least one test specimen for exposure to concentrated solar radiation. A reflector device reflects solar radiation and concentrates the reflective solar radiation on to the target board for illuminating the at least one test specimen. An air circulation device circulates ambient air over the target board for adjusting the temperature of the at least one test specimen. The air circulation device includes an electric motor and a fan powered by the electric motor for creating a flow of ambient air. A feedback device is mounted to the target board for exposure to the concentrated solar radiation and for generating a test signal responsive to the temperature thereof and representative of the test specimen temperature. An input device generates a dynamic reference signal representative of a complex temperature cycle of a material end-use application. A controller connects to the input device and is responsive to the reference signal for generating a dynamic temperature set point and is further connected to the feedback device and is responsive to the test signal for selectively controlling the application of electrical power to the electric motor in order to control a rate at which ambient air is circulated over the target board. The rate is generally increased when the temperature of the feedback device is greater than the dynamic temperature set point, and is generally decreased when the temperature of the feedback device is less than the dynamic temperature set point. The rate is generally maintained constant when the temperature of feedback device is substantially equal to the dynamic temperature set point.
Another aspect of the present invention is directed to an accelerated weathering test apparatus of the type used to concentrate solar radiation upon test specimens where the apparatus is adapted to dynamically control a test specimen temperature in accordance with more than one input device. The apparatus includes a target board for supporting at least one test specimen for exposure to concentrated solar radiation. A reflector reflects solar radiation and concentrates the reflected solar radiation on to the target board for illuminating the at least one test specimen. An air circulation device circulates ambient air over the target board for cooling the at least one test specimen. The air circulation device includes an electric motor and a fan powered by the electric motor for creating a flow of ambient air. At least one feedback device is mounted to the target board for exposure to the concentrated solar radiation and each at least one feedback device generates a respective test signal responsive to the temperature thereof and representative of the test specimen temperature. The apparatus further includes at least two input devices which each generates a respective dynamic reference signal. A controller is connected to a first switch for alternatively selecting one of the at least two input devices and is responsive to the selected reference signal for generating a temperature set point. The controller is further connected to a second switch for alternatively selecting one of the at least one feedback device and is responsive to the selected test signal for selectively controlling the application of electrical power to the electric motor, in order to control the rate at which ambient air is circulated over the target board. The rate is generally increased when the temperature of the selected one of the at least one feedback device is greater than the dynamic temperature set point and the rate is generally decreased when the temperature of the one of the at least one feedback device is less than the dynamic temperature set point. The rate is generally maintained when the temperature of the one of the at least one feedback device is substantially equal to the dynamic temperature set point.
Yet another aspect of the present invention is directed to a system for tightly regulating temperature variability between a plurality of accelerated weathering test apparatus of the type used to concentrate solar radiation upon test specimens during an exposure test. The test specimens on each of the plurality of apparatus do not need to be identical. Each apparatus is adapted to dynamically control a test specimen temperature. The system includes the plurality of accelerated weathering test apparatus each including a target board for supporting at least one test specimen to be exposed to concentrated solar radiation. A reflector for reflecting the solar radiation and concentrating the reflected solar radiation onto the target board for illuminating the at least one test specimen. An air circulation device for circulating ambient air over the target board for cooling the at least one test specimen. The air circulation device includes an electric motor and a fan powered by the electrical motor for creating a flow of ambient air. A feedback device sensor is mounted to the target board for exposure to the concentrated solar radiation and generating a test signal responsive to the temperature thereof and representative of the test specimen temperature. An input device for generating a dynamic reference signal representative of a complex temperature cycle of a material end-use application. A controller is connected to the input device and is responsive to the reference signal for generating a dynamic temperature set point. The controller is further connected to the feedback device and is responsive to the test signal for selectively controlling application of electrical power to the electrical motor in order to control the rate at which ambient air is circulated over the target board. The rate is generally increased when the temperature of the feedback device is greater than the dynamic temperature set point and generally decreased when the temperature of the feedback device is less than the dynamic temperature set point. The rate is generally maintained when the temperature of the feedback device is substantially equal to the dynamic temperature set point. The input device of the first one apparatus is disposed remote from the plurality of accelerated weathering test apparatus. The input device of each other apparatus is consecutively linked in series to the first one apparatus such that the other apparatus are dependently controlled from the first one apparatus to reduce temperature variability across the system.